[PATCH] libbpf: fix double-free of distilled base BTF on .BTF.ext parse error

[Naveed Khan <naveed@digiscrypt.com>]

When btf_parse_elf() is called without a caller-supplied base_btf (i.e.
via the public btf__parse_elf()) and the object file carries a .BTF.base
(distilled base) section, a dist_base_btf object is created and used as
the base of the split BTF built from the .BTF section.  Because base_btf
is NULL, the relocation block that would otherwise free and clear
dist_base_btf is skipped, and ownership of dist_base_btf is instead
transferred to the split btf by setting btf->owns_base = true.

That ownership transfer was performed before the fallible btf_ext__new()
call that parses the .BTF.ext section.  If .BTF.ext is malformed,
btf_ext__new() fails and the function jumps to the error path, which
frees dist_base_btf directly and then frees btf.  Since owns_base is
already set, btf__free(btf) also frees btf->base_btf, which is the same
dist_base_btf object.  The result is a use-after-free read followed by a
double free of the base BTF, driven entirely by a crafted object file
(a .BTF + .BTF.base + malformed .BTF.ext combination) passed to
btf__parse_elf(), as used by bpftool, pahole and similar tools.

Transfer ownership only after .BTF.ext has been parsed successfully, so
that any earlier failure leaves dist_base_btf owned solely by the local
cleanup path and it is freed exactly once.

Signed-off-by: Naveed Khan <naveed@digiscrypt.com>
---
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 823bce8951..35695f33c6 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -1506,9 +1506,6 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 		dist_base_btf = NULL;
 	}
 
-	if (dist_base_btf)
-		btf->owns_base = true;
-
 	switch (gelf_getclass(elf)) {
 	case ELFCLASS32:
 		btf__set_pointer_size(btf, 4);
@@ -1530,6 +1527,9 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 	} else if (btf_ext) {
 		*btf_ext = NULL;
 	}
+
+	if (dist_base_btf)
+		btf->owns_base = true;
 done:
 	if (elf)
 		elf_end(elf);